WORK MACHINE, CONTROLLER FOR WORK MACHINE, AND CONTROL METHOD FOR WORK MACHINE

TECHNICAL FIELD

[0001]The present disclosure relates to a work machine, a controller for a work machine, and a control method for a work machine.

BACKGROUND ART

[0002]A work machine such as a motor grader may be provided with an all-wheel drive device that drives all of the front and rear wheels. When the target speed of the front wheels is controlled to be higher than the speed of the rear wheels and the driving force applied to the front wheels increases while the work machine is traveling with all-wheel drive, the front wheel driving force may exceed the slip limit of the front wheels and a slip of the front wheels may occur. Frequent slips of the front wheel may cause a vibration phenomenon (hopping) in which the front wheels are separated from the ground and come into contact with the ground repeatedly. U.S. Pat. No. 5,474,147A (PTL 1) discloses a technique of reducing a driving force applied to a front wheel when an occurrence of hopping is detected.

CITATION LIST

Patent Literature

[0003]PTL 1: U.S. Pat. No. 5,474,147A

SUMMARY OF INVENTION

Technical Problem

[0004]In an all-wheel-drive-type work machine, the occurrence of hopping can be suppressed by reducing the front wheel driving force, but when the front wheel driving force is lower than a command value given by an operator's operation, work efficiency may decrease.

[0005]The present disclosure proposes a work machine, a controller for a work machine, and a control method for a work machine that can suppress a decrease in work efficiency.

Solution to Problem

[0006]According to an aspect of the present disclosure, a work machine is proposed, the work machine including a front wheel, a front wheel drive device that rotationally drives the front wheel, a rear wheel, a rear wheel drive device that rotationally drives the rear wheel, and a controller that controls a speed ratio that is a ratio of a speed of the front wheel to a speed of the rear wheel. The controller determines whether a predetermined variation occurs in a driving force applied to the front wheel, and when it is determined that the predetermined variation occurs, the controller reduces the speed ratio and thus eliminates the predetermined variation, and when the predetermined variation is eliminated, the controller increases the speed ratio.

[0007]According to an aspect of the present disclosure, a controller for a work machine is proposed. The controller determines whether a predetermined variation occurs in a driving force applied to a front wheel of the work machine. When it is determined that the predetermined variation occurs, the controller reduces a speed ratio that is a ratio of a speed of the front wheel to a speed of a rear wheel of the work machine and thus eliminates the predetermined variation. When the predetermined variation is eliminated, the controller increases the speed ratio.

[0008]According to an aspect of the present disclosure, a control method for a work machine is proposed. The control method includes the following steps. A first step is to determine whether a predetermined variation occurs in a driving force applied to a front wheel of the work machine. A second step is to reduce a speed ratio that is a ratio of a speed of the front wheel to a speed of a rear wheel of the work machine and thus eliminate the predetermined variation, when it is determined that the predetermined variation occurs. A third step is to increase the speed ratio when the predetermined variation is eliminated.

Advantageous Effects of Invention

[0009]According to the work machine, the controller for the work machine, and the control method for the work machine according to the present disclosure, a decrease in work efficiency can be suppressed.

BRIEF DESCRIPTION OF DRAWINGS

[0010]FIG. 1 is a side view schematically showing the configuration of a motor grader in an embodiment.

[0011]FIG. 2 is a configuration diagram showing a schematic configuration of the motor grader shown in FIG. 1.

[0012]FIG. 3 is a block diagram illustrating the functional configuration of a controller.

[0013]FIG. 4 is a flowchart showing a flow of processing of travel control of the motor grader in the embodiment.

[0014]FIG. 5 is a graph showing an example of a waveform of a pressure of hydraulic oil when hopping occurs during forward movement.

[0015]FIG. 6 is a diagram illustrating transition of a hunting determination state.

[0016]FIG. 7 is a flowchart illustrating a flow of processing of hopping suppression control.

[0017]FIG. 8 is a table showing an example of the relationship between a speed ratio adjustment dial effective value (speed ratio adjustment dial command value) and a speed ratio.

[0018]FIG. 9 is a flowchart illustrating a flow of processing of forward driving force recovery control.

DESCRIPTION OF EMBODIMENTS

[0019]Hereinafter, an embodiment will be described with reference to the drawings. In the description below, the same components and elements are denoted by the same reference numerals. The names and functions thereof are also the same. Therefore, detailed description thereof will not be repeated. It is also planned from the beginning that any configurations are extracted from the embodiment and freely combined together.

[0020]In the embodiment, a motor grader 1 will be described as an example of a work machine. FIG. 1 is a side view schematically showing the configuration of the motor grader 1 in the embodiment.

[0021]As shown in FIG. 1, the motor grader 1 in the embodiment is a vehicle having six wheels in total. The motor grader 1 includes travel wheels including a pair of left and right front wheels and two rear wheels on each side. The front wheels include a left front wheel 2 and a right front wheel, not shown in FIG. 1. The rear wheels include a left rear front wheel 4, a left rear rear wheel 5, and a right rear front wheel and a right rear rear wheel, not shown. The number and arrangement of front wheels and rear wheels are not limited to the example shown in FIG. 1.

[0022]The motor grader 1 includes a work implement including a blade 50. The blade 50 is provided between the front wheels and the rear wheels. The motor grader 1 can perform work such as ground leveling work, snow removal work, and light cutting, with the blade 50.

[0023]The motor grader 1 includes a vehicle body frame. The vehicle body frame includes a front frame 51 and a rear frame 52. The front frame 51 is coupled to the rear frame 52 so as to be rotationally movable.

[0024]The front wheels are provided on the front frame 51, together with the blade 50. The front wheels are rotatably attached to a front end portion of the front frame 51. The rear wheels are provided on the rear frame 52. The rear wheels are attached to the rear frame 52 so as to be rotationally driven by a driving force from an engine as described later.

[0025]FIG. 2 is a configuration diagram showing a schematic configuration of the motor grader 1 shown in FIG. 1. The above-described pair of left and right front wheels include the left front wheel 2 and a right front wheel 3. The motor grader 1 includes an engine 6. The engine 6 is supported by the rear frame 52 shown in FIG. 1. The engine 6 is a drive source that generates a driving force for rotationally driving the front wheels and the rear wheels, and is, for example, a diesel engine.

[0026]The left rear wheels 4, 5 and right rear wheels (not shown) paired with the left rear wheels 4, 5 are coupled to the output side of the engine 6 via a torque converter 8, a transmission 9, a final reduction gear device 10, and a tandem device 11. The torque converter 8 is a fluid clutch that transmits a driving force from the engine 6, using oil as a medium. The transmission 9 is a mechanical transmission. The transmission 9 includes a plurality of clutches corresponding to a plurality of speed stages. The transmission 9 switches the connected state and the disconnected state of each clutch and thus switches the speed stage between a plurality of stages. The engine 6 drives the left rear wheels 4, 5 and the right rear wheels via the torque converter 8, the transmission 9, the final reduction gear device 10, and the tandem device 11.

[0027]The torque converter 8, the transmission 9, the final reduction gear device 10, and the tandem device 11 form a rear wheel power transmission device that transmits the driving force generated by the engine 6 to the rear wheels. The final reduction gear device 10 is equivalent to an example of a rear wheel drive device that rotationally drives the rear wheels.

[0028]A pair of left and right hydraulic systems 7L, 7R are coupled to the transmission 9. The hydraulic system 7L drives the left front wheel 2. The hydraulic system 7R drives the right front wheel 3. The engine 6 drives the left front wheel 2 and the right front wheel 3 via the torque converter 8, the transmission 9, and the hydraulic systems 7L, 7R. The hydraulic systems 7L, 7R may be coupled to the other output side of the engine 6 without via the mechanical transmission 9. Each of the left and right hydraulic systems 7L, 7R forms a hydraulic static transmission (HST).

[0029]The motor grader 1 is an all-wheel drive vehicle in which all of the front wheels 2, 3, the left rear wheels 4, 5, and the right rear wheels can be driven by the devices 6 to 11 for power generation and transmission. The devices 6 to 11 form an all-wheel drive device 12. The engine 6, a part of the hydraulic systems 7L, 7R, the torque converter 8, the transmission 9, and the final reduction gear device 10, of the all-wheel drive device 12, are supported by the rear frame 52.

[0030]The hydraulic system 7L includes a left hydraulic pump 15 and a left hydraulic motor 16. The hydraulic system 7R includes a right hydraulic pump 17 and a right hydraulic motor 18. The output of the engine 6 is transmitted to the left hydraulic pump 15 and the right hydraulic pump 17 via a power take-off (PTO) 14, and the left hydraulic pump 15 and the right hydraulic pump 17 are driven. The left hydraulic motor 16 is rotated by the hydraulic oil discharged from the left hydraulic pump 15 and drives the left front wheel 2. The right hydraulic motor 18 is rotated by the hydraulic oil discharged from the right hydraulic pump 17 and drives the right front wheel 3.

[0031]The hydraulic pumps 15, 17 are variable displacement hydraulic pumps. The hydraulic pumps 15, 17 may be a swash plate hydraulic pump having a variable swash plate. The angle of the variable swash plate of the left hydraulic pump 15 is continuously and steplessly controlled by a swash plate drive unit 15A in accordance with a control command value output from a controller, described later. The angle of the variable swash plate of the right hydraulic pump 17 is continuously and steplessly controlled by a swash plate drive unit 17A independently of the variable swash plate of the left hydraulic pump 15 in accordance with a control command value output from a controller, described later. The swash plate drive units 15A, 17A are, for example, solenoids.

[0032]The hydraulic motors 16, 18 may be variable displacement motors. The hydraulic motors 16, 18 may be bent axial motors. The displacement of the hydraulic motors 16, 18 becomes a predetermined value depending on the speed stage selected by the operator. The hydraulic motors 16, 18 may be fixed displacement motors.

[0033]The left hydraulic pump 15 and the left hydraulic motor 16 are coupled by a left hydraulic circuit 21. The hydraulic oil discharged from the left hydraulic pump 15 is supplied to the left hydraulic motor 16 via the left hydraulic circuit 21. The rotation speed of the left front wheel 2 when the left front wheel 2 is driven is controlled by the hydraulic oil discharged from the left hydraulic pump 15. The left hydraulic circuit 21 is provided with pressure sensors 27L, 28L that detect the pressure of the hydraulic oil in the left hydraulic circuit 21. The pressure sensors 27L, 28L output signals indicating the hydraulic pressure in the left hydraulic circuit 21.

[0034]The right hydraulic pump 17 and the right hydraulic motor 18 are coupled by a right hydraulic circuit 22. The hydraulic oil discharged from the right hydraulic pump 17 is supplied to the right hydraulic motor 18 via the right hydraulic circuit 22. The rotation speed of the right front wheel 3 when the right front wheel 3 is driven is controlled by the hydraulic oil discharged from the right hydraulic pump 17. The right hydraulic circuit 22 is provided with pressure sensors 27R, 28R that detect the pressure of the hydraulic oil in the right hydraulic circuit 22. The pressure sensors 27R, 28R output signals indicating the hydraulic pressure in the right hydraulic circuit 22.

[0035]In the power transmission device for transmitting the driving force from the engine 6 to the front wheels 2, 3, the hydraulic pumps 15, 17 are driven by the engine 6 to generate pressure in the hydraulic oil, and the hydraulic motors 16, 18 are driven by the pressure oil discharged from the hydraulic pumps 15, 17 to generate the rotational force again. That is, as described above, each of the left and right hydraulic systems 7L, 7R forms an HST.

[0036]The pressure sensors 27L, 27R are provided in an oil passage through which hydraulic oil flows from the hydraulic pump to the hydraulic motor when the motor grader 1 travels forward. The pressure sensors 27L, 27R detect the pressure of high-pressure hydraulic oil discharged from the hydraulic pump when the motor grader 1 travels forward. The pressure sensors 28L, 28R are provided in an oil passage through which hydraulic oil flows from the hydraulic pump to the hydraulic motor when the motor grader 1 travels backward. The pressure sensors 28L, 28R detect the pressure of high-pressure hydraulic oil discharged from the hydraulic pump when the motor grader 1 travels backward.

[0037]A left hydraulic clutch mechanism 23 and a left speed reducer 25 are provided between the left front wheel 2 and the left hydraulic motor 16. A right hydraulic clutch mechanism 24 and a right speed reducer 26 are provided between the right front wheel 3 and the right hydraulic motor 18. When hydraulic pressure is supplied to the left hydraulic clutch mechanism 23 and the right hydraulic clutch mechanism 24, power is transmitted to the left front wheel 2 and the right front wheel 3, and the motor grader 1 is driven on all wheels. When the supply of the hydraulic pressure to the left hydraulic clutch mechanism 23 and the right hydraulic clutch mechanism 24 is cut off, the all-wheel drive of the motor grader 1 is canceled and the motor grader 1 is driven on the rear wheels.

[0038]The torque converter 8, the transmission 9, the PTO 14, the hydraulic systems 7L, 7R, the clutch mechanisms 23, 24, and the speed reducers 25, 26 form a front wheel power transmission device that transmits the driving force generated by the engine 6 to the front wheels. The hydraulic motors 16, 18 are equivalent to an example of a front wheel drive device that rotationally drives the front wheels. The rotation speed of the front wheels can be increased by increasing the supply amount of the hydraulic oil supplied from the hydraulic pumps 15, 17 to the hydraulic motors 16, 18 (discharge amount of the hydraulic pump). The rotation speed of the front wheels can be reduced by reducing the supply amount of the hydraulic oil supplied from the hydraulic pumps 15, 17 to the hydraulic motors 16, 18.

[0039]A speed sensor 31 is provided on the output shaft of the transmission 9. The speed sensor 31 measures a rotation speed of the output shaft of transmission 9 and thus detects the rotation speed of the rear wheels during the movement (traveling) of the motor grader 1. The speed sensor 31 outputs a signal indicating the rotation speed of the rear wheels.

[0040]FIG. 3 is a block diagram illustrating the functional configuration of a controller 60. The motor grader 1 includes the controller 60. The controller 60 reads and executes various programs by a central processing unit (CPU), not shown, and performs various arithmetic processing.

[0041]A speed ratio adjustment dial 40 is operated by the operator. The amount of operation of the speed ratio adjustment dial 40 by the operator is converted into an electric signal and input to the controller 60. The speed ratio is the ratio of the speed of the front wheel to the speed of the rear wheel. When the operator operates the speed ratio adjustment dial 40, a manually set value of the speed ratio (hereinafter referred to as a speed ratio adjustment dial command value) is input to a dial command value input unit 61 of the controller 60. The dial command value input unit 61 accepts an input of a command value of the speed ratio by the operator's operation.

[0042]The speed ratio adjustment dial 40 in the embodiment can be set, for example, to seven stages. By operating the speed ratio adjustment dial 40, the operator can selectively set the speed ratio adjustment dial command value to any one of integers of 1 or greater and 7 or smaller, and switch the speed ratio, which is the ratio of the speed of the front wheel to the speed of the rear wheel, between seven stages (see FIG. 8).

[0043]The pressure sensor 27L provided in the left hydraulic circuit 21 and the pressure sensor 27R provided in the right hydraulic circuit 22 shown in FIG. 2 are collectively referred to as a pressure sensor 27. The pressure sensor 28L provided in the left hydraulic circuit 21 and the pressure sensor 28R provided in the right hydraulic circuit 22 are collectively referred to as a pressure sensor 28. The result of detection of the pressure of the hydraulic oil in the hydraulic circuit is input from the pressure sensors 27, 28 to a pressure detection value input unit 63 of the controller 60. The result of detection of the rotation speed of the output shaft of the transmission 9 is output from the speed sensor 31 to a motor rotation speed calculation unit 70 of the controller 60.

[0044]The controller 60 includes a memory 72. The memory 72 stores a program for controlling the operation of the motor grader 1 and various data necessary for the execution of the program. The memory 72 also temporarily stores working data generated as the work is executed. The controller 60 includes a timer 73. The timer 73 measures time.

[0045]FIG. 4 is a flowchart showing a flow of processing of travel control of the motor grader 1 in the embodiment. Hereinafter, processing in which the controller 60 automatically controls the speed ratio will be described with reference to FIGS. 3 and 4 and subsequent drawings as appropriate.

[0046]As shown in FIG. 4, in step S1, the controller 60 determines whether the speed ratio adjustment dial 40 is updated. The dial command value input unit 61 receives an input of a speed ratio adjustment dial command value from the speed ratio adjustment dial 40. A dial update determination unit 62 reads a stored speed ratio adjustment dial command value from the memory 72. As will be described later, the memory 72 stores the speed ratio adjustment dial command value at a time when it is previously determined that the speed ratio adjustment dial 40 is updated.

[0047]The dial update determination unit 62 compares the speed ratio adjustment dial command value read from the memory 72 with the speed ratio adjustment dial command value currently input from the speed ratio adjustment dial 40. When the speed ratio adjustment dial command value stored in the memory 72 is different from the current speed ratio adjustment dial command value as a result of comparing the speed ratio adjustment dial command values, the dial update determination unit 62 determines that the speed ratio adjustment dial 40 is updated.

[0048]If it is determined in step S1 that the speed ratio adjustment dial 40 is updated, the dial command value input unit 61 stores the current speed ratio adjustment dial command value in the memory 72. Thus, the memory 72 constantly stores the speed ratio adjustment dial command value that is the latest command value manually set by the operator.

[0049]It is determined that the speed ratio adjustment dial 40 is updated (YES in step S1), when the operator operates the speed ratio adjustment dial 40 and the manually set speed ratio adjustment dial command value is changed. If YES in step S1, the processing proceeds to step S2, and a dial effective value setting unit 68 sets the current speed ratio adjustment dial command value as a speed ratio adjustment dial effective value.

[0050]The speed ratio adjustment dial effective value is a set value for controlling the speed ratio adjustment dial 40, which is used to determine the speed ratio. As will be described in detail later, the speed ratio adjustment dial effective value may be the same as the speed ratio adjustment dial command value or may be lower than the speed ratio adjustment dial command value, depending on conditions. If the speed ratio adjustment dial command value can set seven stages of speed ratios via the speed ratio adjustment dial 40, the speed ratio adjustment dial effective value is also selectively set from among seven stages.

[0051]FIG. 8 is a table showing an example of the relationship between the speed ratio adjustment dial effective value (speed ratio adjustment dial command value) and the speed ratio. In the example shown in FIG. 8, the speed ratio is 1.00 when the speed ratio adjustment dial effective value (speed ratio adjustment dial command value) is 3. As the speed ratio adjustment dial effective value (speed ratio adjustment dial command value) increases from 3 by 1 each, the speed ratio increases by 0.05 each. When the speed ratio adjustment dial effective value (speed ratio adjustment dial command value) is 7, the speed ratio is 1.20. As the speed ratio adjustment dial effective value (speed ratio adjustment dial command value) decreases from 3 by 1 each, the speed ratio decreases by 0.05 each. When the speed ratio adjustment dial effective value (speed ratio adjustment dial command value) is 1, the speed ratio is 0.90.

[0052]The speed ratio is the ratio of the speed of the front wheel to the speed of the rear wheel. When the speed ratio is higher than 1, the speed of the front wheel is greater than the speed of the rear wheel. When the speed ratio is 1, the speed of the rear wheel and the speed of the front wheel are the same. When the speed ratio is lower than 1, the speed of the front wheel is lower than the speed of the rear wheel.

[0053]Referring back to FIG. 4, if it is determined that the speed ratio adjustment dial 40 is not updated (NO in step S1), the processing proceeds to step S3, and a hunting determination unit 64 determines whether hunting of the pressure in the HST circuit occurs.

[0054]FIG. 5 is a graph showing an example of a waveform of the pressure of the hydraulic oil when hopping occurs during forward movement. The horizontal axis of the graph shown in FIG. 5 represents time, and the HST circuit pressure on the vertical axis represents the pressure of the hydraulic oil in the hydraulic circuits 21, 22 detected by the pressure sensors 27, 28 and input to the pressure detection value input unit 63. In FIG. 5, a variation with time in the pressure of the hydraulic oil in one of the left hydraulic circuit 21 and the right hydraulic circuit 22 is illustrated.

[0055]As shown in FIG. 5, when the front wheels 2, 3 slip on the road surface while the front wheels 2, 3 are gripping the road surface, the HST circuit pressure rapidly decreases. In the example shown in FIG. 5, the HST circuit pressure at time t_n-dt is P_HST(tn-dt), the HST circuit pressure at time t_n is P_HST(tn), and the HST circuit pressure rapidly decreases during the time dt. In the present embodiment, the hunting determination unit 64 detects the slip of the front wheels 2, 3, based on the variation in the HST circuit pressure input to the pressure detection value input unit 63. The hunting determination unit 64 determines that the hopping of the front wheels occurs when it is detected that the slip of the front wheels 2, 3 is repeated a predetermined number of times or more within a certain time (hunting of the HST circuit pressure occurs).

[0056]FIG. 6 is a diagram illustrating the transition of the hunting determination state. Referring to FIGS. 5 and 6, further details of the determination of hunting of the pressure of the hydraulic oil will be described.

[0057]When the hunting determination is started (“START” in FIG. 6), the hunting determination unit 64 sets a state of “no hunting” and sets a slip count (described later) to 0. When a condition B (described later) is satisfied and a condition C (described later) is satisfied in the state of “no hunting”, the hunting determination unit 64 determines that the state is “hunting determination in progress”. The condition B is that a mask time tm elapses from the time when “no hunting” is determined. The condition C is that the change rate of the HST circuit pressure within the predetermined time dt is higher than a threshold. Specifically, the change rate of the HST circuit pressure is a decrease rate of the HST circuit pressure when in the forward movement shown in FIG. 5, and is an increase rate of the HST circuit pressure when in the backward movement.

[0058]The transition from the state of “no hunting” to the state of “hunting determination in progress” is triggered by the variation in the HST circuit pressure. When a sudden decrease in the HST circuit pressure, that is, a slip of the front wheels 2, 3 is detected once, the state transitions to the “hunting determination in progress” state.

[0059]When the state transitions to the “hunting determination in progress” state, the hunting determination unit 64 starts counting time. The hunting determination unit 64 reads the current time from the timer 73. As shown in FIG. 5, the hunting determination unit 64 starts counting time, starting from time t_n at which a rapid decrease in the HST circuit pressure related to satisfaction of the condition C is detected.

[0060]When the state transitions to the “hunting determination in progress” state, the hunting determination unit 64 also counts the number of times the HST circuit pressure rapidly decreases, and stores the count as the slip count.

[0061]The memory 72 of the controller 60 stores a predetermined hunting determination execution upper limit time. When the slip count reaches a predetermined number of times (three times in the present embodiment) before the counted time exceeds the hunting determination execution upper limit time, the hunting determination unit 64 causes the state to transition from the “hunting determination in progress” state to a “hunting occurring” state. Meanwhile, when a condition D (described later) is satisfied in the state of “hunting determination in progress”, the hunting determination unit 64 causes the state to transition to the state of “no hunting”. The condition D is that the hunting determination execution upper limit time elapses while the number of times of slip detection does not reach the number of times of hunting determination.

[0062]In the present embodiment, the number of times of hunting determination is set to three, and in the example shown in FIG. 5, since the rapid decrease of the third HST circuit pressure is detected before the hunting determination execution upper limit time elapses, the hunting determination unit 64 causes the state to transition from the “hunting determination in progress” state to the “hunting occurring” state.

[0063]As shown in FIG. 6, when the state transitions to the state of “hunting occurring”, the controller 60 performs hopping suppression control and unconditionally causes the state of the hunting determination to transition to “no hunting”.

[0064]When the condition D is satisfied in the state of “hunting determination in progress” or when it is determined that “hunting is occurring” and the state transitions to the state of “no hunting”, the slip count is reset to zero. Thereafter, the state of “no hunting” is continuously determined until the condition B is satisfied, that is, until the mask time tm elapses. In consideration of the response of the hopping suppression control described below, the slip detection of the front wheels 2, 3 is not performed until the mask time tm elapses.

[0065]Referring back to FIG. 4, when the hunting determination unit 64 determines “hunting determination in progress” in the determination of step S3, the dial effective value setting unit 68 maintains the previous speed ratio adjustment dial effective value in step S4.

[0066]When the hunting determination unit 64 determines “hunting occurring” in the determination of step S3, the controller 60 executes the hopping suppression control in step S5. When the hunting determination unit 64 determines “hunting occurring” in either the left front wheel 2 or the right front wheel 3, the controller 60 changes the speed ratio adjustment dial effective value as a measure for reducing the speed ratio of the front wheels 2, 3 to the rear wheels in order to suppress the slip of the front wheels 2, 3.

[0067]FIG. 7 is a flowchart illustrating a flow of processing of the hopping suppression control. In the example shown in FIG. 7, the speed ratio adjustment dial effective value can be set selectively to any one of integers of 1 or greater and 7 or smaller (in seven stages), similarly to the speed ratio adjustment dial command value that can be set by the speed ratio adjustment dial 40 described above (see FIG. 8).

[0068]Here, in the present embodiment, the speed ratio increases as the speed ratio adjustment dial effective value increases, and the speed ratio is set to 1 (the speeds of the front wheels and the rear wheels are the same) when the speed ratio adjustment dial effective value is 3.

[0069]Referring back to FIG. 7, in step S11, a dial effective value determination unit 67 determines the current speed ratio adjustment dial effective value.

[0070]If the current speed ratio adjustment dial effective value is 4 or higher and 7 or lower, the dial effective value setting unit 68 changes the speed ratio adjustment dial effective value to 3 in step S14. If the current speed ratio adjustment dial effective value is 2 or 3, the dial effective value setting unit 68 performs processing of decreasing (decrementing) the speed ratio adjustment dial effective value by 1 in step S13. If the current speed ratio adjustment dial effective value is 1, the dial effective value setting unit 68 maintains the speed ratio adjustment dial effective value at 1 in step S12. In this way, the hopping suppression control is executed (“END” in FIG. 7).

[0071]If the speed ratio adjustment dial effective value at a time when “hunting occurring” is determined is 4 or higher and the speed ratio is higher than 1, the dial effective value setting unit 68 sets the speed ratio adjustment dial effective value to 3 and thus decreases the speed ratio to 1. If the speed ratio adjustment dial effective value at a time when “hunting occurring” is determined is 2 or 3 and the speed ratio is 1 or lower, the dial effective value setting unit 68 decreases the speed ratio stepwise by one stage each. As shown in FIG. 8, in the present embodiment, the dial effective value setting unit 68 decrements the speed ratio adjustment dial effective value by 1 and thus decreases the speed ratio by 0.05. If the speed ratio adjustment dial effective value at a time when “hunting occurring” is determined is 1 and the speed ratio is a settable minimum value, the dial effective value setting unit 68 maintains the speed ratio adjustment dial effective value at 1 and maintains the speed ratio at the minimum value.

[0072]Referring back to FIG. 4 again, when the hunting determination unit 64 determines “no hunting” in the determination of step S3, the controller 60 executes front wheel driving force recovery control in step S6.

[0073]FIG. 9 is a flowchart illustrating a flow of processing of forward driving force recovery control. In step S21, a dial value comparison unit 65 reads the speed ratio adjustment dial command value from the memory 72. The dial effective value determination unit 67 determines the current speed ratio adjustment dial effective value. The dial value comparison unit 65 compares the speed ratio adjustment dial command value read from the memory 72 with the current speed ratio adjustment dial effective value, and determines whether there is a deviation between the speed ratio adjustment dial command value and the speed ratio adjustment dial effective value. Specifically, the dial value comparison unit 65 determines whether the current speed ratio adjustment dial effective value is lower than the speed ratio adjustment dial command value.

[0074]If it is determined that the speed ratio adjustment dial effective value is lower than the speed ratio adjustment dial command value (YES in step S21), a time determination unit 66 reads the current time from the timer 73 in step S22. The time determination unit 66 calculates an elapsed time from the time when “no hunting” is determined to the current time. The time determination unit 66 determines whether the state of “no hunting” continues for a predetermined time tr.

[0075]If it is determined that the state of “no hunting” continues for time tr (YES in step S22), the dial effective value setting unit 68 performs processing (incrementing) of adding 1 to the speed ratio adjustment dial effective value in step S23. By this processing, the speed ratio increases by 0.05 each, as shown in FIG. 8.

[0076]If it is determined in step S21 that the speed ratio adjustment dial effective value is equal to or higher than the speed ratio adjustment dial command value (NO in step S21) or it is determined in step S22 that the state of “no hunting” does not continue for the time tr (NO in step S22), the dial effective value setting unit 68 maintains the previous speed ratio adjustment dial effective value in step S24. In this way, the front wheel driving force recovery control is executed (“END” in FIG. 9).

[0077]The time tr, which is the threshold in the determination of step S22, is set to be longer than the mask time tm, which is the threshold in the hunting determination condition B shown in FIG. 6. For example, the time tr may be four times the mask time tm. The time tr may be set to be a time longer than the hunting determination execution upper limit time.

[0078]When the speed ratio adjustment dial effective value reaches the speed ratio adjustment dial command value by the dial effective value setting unit 68 incrementing the speed ratio adjustment dial effective value by 1 each, the dial value comparison unit 65 determines NO in the determination in step S21, and subsequently the speed ratio adjustment dial effective value is maintained. The dial effective value setting unit 68 increases the speed ratio adjustment dial effective value within a range not exceeding the speed ratio adjustment dial command value.

[0079]Referring back to FIG. 4 again, in step S7, a speed ratio determination unit 69 collates the speed ratio adjustment dial effective value set in step S2, step S4, steps S12 to S14, or steps S23, S24 with the table shown in FIG. 8, and thus determines the speed ratio.

[0080]In step S8, the hydraulic pumps 15, 17 are controlled. The motor rotation speed calculation unit 70 calculates the speed of the rear wheels, based on the detection value of the speed sensor 31. The motor rotation speed calculation unit 70 multiplies the speed of the rear wheels by the speed ratio and thus calculates a set value of the speed of the front wheels 2, 3. The motor rotation speed calculation unit 70 calculates a target rotation speed of the hydraulic motors 16, 18 required to rotationally drive the front wheels 2, 3 at that speed.

[0081]A pump displacement command unit 71 calculates the flow rate of the hydraulic oil for rotating the hydraulic motors 16, 18 at the target rotation speed. The pump displacement command unit 71 transmits a control signal to the swash plate drive units 15A, 17A of the respective hydraulic pumps 15, 17 so that the hydraulic oil is supplied from the hydraulic pumps 15, 17 to the hydraulic motors 16, 18 at that flow rate. Even when the rotation speed of the engine 6 is the same, the flow rate of the hydraulic oil discharged from the hydraulic pumps 15, 17 can be changed by controlling the swash plates of the respective hydraulic pumps 15, 17. In this manner, the hydraulic pumps 15, 17 are controlled.

[0082]When the speed ratio is higher than 1, the hydraulic pumps 15, 17 are controlled to rotate the front wheels at a speed higher than that of the rear wheels.

[0083]When the series of processes shown in FIG. 4 is finished, the processing is returned (“RETURN” in FIG. 4), and the processing of setting the speed ratio adjustment dial effective value in accordance with the traveling state of the motor grader 1 is repeated. When the operator does not operate the speed ratio adjustment dial 40 (when the speed ratio adjustment dial command value is not updated), the hunting determination is executed again, and the hopping suppression control shown in FIG. 7 is executed when “hunting occurring” is determined, and the front wheel driving force recovery control shown in FIG. 9 is executed when “no hunting” is determined.

[0084]As described with reference to FIG. 6, after “hunting occurring” is determined, the state of the hunting determination transitions to “no hunting” unconditionally, and the state of “no hunting” is maintained until the condition B is satisfied, that is, until the mask time tm elapses. Therefore, in the next determination of step S3 after the return in the flowchart of FIG. 4, the state is “no hunting”.

[0085]The time tr in the determination of step S22 in FIG. 9 is set to be longer than the mask time tm, which is the threshold in the hunting determination condition B. Thus, frequent repetitions of decrease and increase in the speed ratio adjustment dial effective value are avoided and stable control is realized.

[0086]That is, the speed ratio adjustment dial effective value is maintained in step S24 until the mask time tm elapses. If the hunting of the pressure of the hydraulic oil is still occurring after the speed ratio adjustment dial effective value is decreased in the hopping suppression control (step S5, FIG. 7), a sudden change in the pressure of the hydraulic oil can be detected before the time tr is reached after the lapse of the mask time tm. A transition is made from the state of “no hunting” to the state of “hunting determination in progress”, and “hunting determination in progress” is determined in the determination of step S3, and the speed ratio adjustment dial effective value is maintained in step S4.

[0087]As described above, in the present embodiment, as shown in FIG. 4 to 6, the controller 60 (hunting determination unit 64) determines whether hunting occurs in the pressure of the hydraulic oil supplied to the hydraulic motors 16, 18. The hydraulic motors 16, 18 are front wheel drive devices that rotationally drive the front wheels 2, 3, and the pressure of the hydraulic oil supplied to the hydraulic motors 16, 18 is equivalent to an example of the driving force applied to the front wheels 2, 3.

[0088]As shown in FIGS. 4 and 7, when it is determined that hunting occurs in the pressure of the hydraulic oil supplied to hydraulic motors 16, 18, the controller 60 (dial effective value setting unit 68) decreases the speed ratio adjustment dial effective value. Thus, the speed ratio decreases as shown in FIG. 8. As the ratio of the speed of the front wheels 2, 3 to the speed of the rear wheels is reduced and the driving force of the front wheels 2, 3 is thus reduced, the slip of the front wheels 2, 3 is suppressed. Accordingly, the hopping of the front wheels 2, 3 is suppressed, and thus the occurrence of pitching behavior in which the front end portion of the front frame 51 of the motor grader 1 vibrates up and down can be suppressed.

[0089]As shown in FIGS. 4 and 9, when it is determined that the hunting of the pressure of the hydraulic oil supplied to hydraulic motors 16, 18 is eliminated, the controller 60 (dial effective value setting unit 68) increases the speed ratio adjustment dial effective value within the range of the speed ratio adjustment dial command value set by the operator. Thus, the speed ratio increases as shown in FIG. 8. As the speed ratio adjustment dial effective value is brought close to the speed ratio adjustment dial command value by the operator's operation, the driving force of the front wheels 2, 3 increases and approaches the driving force intended by the operator. Thus, work efficiency of the motor grader 1 can be improved.

[0090]As shown in FIG. 9, the controller 60 (dial effective value setting unit 68) increases the speed ratio adjustment dial effective value by 1 each. As a result, as shown in FIG. 8, the speed ratio increases by 0.05 each. When it is determined that the hunting of the pressure of the hydraulic oil supplied to the hydraulic motors 16, 18 is eliminated, the speed ratio adjustment dial effective value is controlled to approach the speed ratio adjustment dial command value stepwise instead of changing the speed ratio adjustment dial effective value to the speed ratio adjustment dial command value at once. As the amount of control is not suddenly changed, the re-occurrence of hopping can be prevented and the motor grader 1 can stably travel.

[0091]As shown in FIG. 3, the controller 60 (dial command value input unit 61) accepts the input of the speed ratio adjustment dial command value by the operator's operation. As shown in FIG. 9, the controller 60 (dial effective value setting unit 68) increases the speed ratio adjustment dial effective value within a range not exceeding the speed ratio adjustment dial command value. As the speed ratio adjustment dial effective value is converged to the speed ratio adjustment dial command value, the situation where the driving force of the front wheels 2, 3 become larger than the operator's intention can be securely avoided.

[0092]As shown in FIGS. 7 and 8, when it is determined that hunting occurs in the pressure of the hydraulic oil supplied to the hydraulic motors 16, 18, the controller 60 (dial effective value setting unit 68) sets the speed ratio adjustment dial effective value to 3 and sets the speed ratio to 1, if the speed ratio adjustment dial effective value is 4 or higher and the speed ratio is higher than 1. When the speed ratio adjustment dial effective value at a time when it is determined that hunting occurs is 5, 6 or 7, the speed ratio adjustment dial effective value is changed to 3 at once, skipping the effective value therebetween. Thus, the hopping of the front wheels 2, 3 can be eliminated in a short time.

[0093]As shown in FIGS. 7 and 8, when it is determined that hunting occurs in the pressure of the hydraulic oil supplied to the hydraulic motors 16, 18, if the speed ratio adjustment dial effective value is 2 or 3 and the speed ratio is 1 or lower, the controller 60 (dial effective value setting unit 68) decreases the speed ratio adjustment dial effective value stepwise by 1 each and decreases the speed ratio stepwise by 0.05 each. Thus, the elimination of the hopping of the front wheels 2, 3 can be facilitated.

[0094]As shown in FIGS. 7 and 8, when it is determined that hunting occurs in the pressure of the hydraulic oil supplied to the hydraulic motors 16, 18, the controller 60 (dial effective value setting unit 68) maintains the speed ratio adjustment dial effective value at 1 and maintains the speed ratio at the minimum value, if the speed ratio adjustment dial effective value is 1 and the speed ratio is the settable minimum value. Thus, the hopping of the front wheels 2, 3 can be more securely eliminated.

[0095]The control for automatically adjusting the speed ratio described in the above embodiment may not be executed on all the speed stages of the transmission. For example, control may be performed such that the speed ratio is automatically adjusted during traveling at a low speed stage, and the speed ratio is not automatically adjusted during traveling at a high speed stage.

[0096]In the above embodiment, power is transmitted to the front wheels 2, 3 by the HST, whereas mechanical power is transmitted to the rear wheels via the transmission 9. The power transmission device is not limited to this example and can be freely selected as long as the rear wheels are rotationally driven by the rear wheel drive device and the front wheels are rotationally driven by the front wheel drive device independently of the rear wheels. For example, power may be transmitted to the rear wheels via an HST different from the hydraulic systems 7L, 7R.

[0097]The controller for setting the speed ratio between the front wheels and the rear wheels of the motor grader 1 described in the above embodiment may not necessarily be installed in the motor grader 1. A system in which the controller 60 installed in the motor grader 1 performs the processing of transmitting the detection values of the pressure sensors 27, 28 to an external controller and in which the external controller having received the signals sets a speed ratio may be configured. The external controller may be disposed at the work site of the motor grader 1 or may be disposed at a remote location away from the work site of the motor grader 1.

[0098]In the above embodiment, the motor grader 1 is described as an example of the work machine, but the present invention is not limited to the motor grader 1 and can also be applied to other types of work machines.

[0099]While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

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